There has been a rapidly increasing demand for dichlorinated toluenes for use as both high boiling solvents and as intermediates in the preparation of pharmaceuticals, dye stuffs, rubber chemicals, and other organic compounds. Especially desirable is the 2,4-dichlorotoluene isomer, as compared to the less desirable 3,4-dichlorotoluene and 2,5-dichlorotoluene isomers. 2,4-dichlorotoluene has been shown to be especially suitable as an intermediate in the formation of herbicides, such as those under U.S. Pat. No. 3,617,252, having qualities which are more compatible with man's increased awareness of environmental and ecological stabilization. Of primary ecological importance is that lesser quantities can be used, to the same herbicidal effect of popular commercial herbicides, thereby causing less pollution. In view of these advantages, commercial output has been projected as increasing substantially in the coming years to production rates in excess of several million pounds per year. Accordingly, improvements to the process of preparing 2,4-dichlorotoluene in higher yields and with more conveniently available commercial raw materials has taken on a greater importance.
Heretofore, dichlorotoluenes have been produced by chloriuating toluene or parachlorotoluene in the presence of iron, zirconium tetrachloride or other known catalysts to form a chlorotoluene product that contains 2,4-dichlorotoluene along with substantial amounts of the less desirable 3,4-dichlorotoluene and 2,5-dichlorotoluene isomers, and quantities of trichlorotoluene. Typically, as chlorination proceeds beyond the stage of mono-chlorination, complex mixtures are produced in unequal proportions. Each catalyst reacts differently under the same or different reaction conditions so that catalytic results become impossible to predict. U.S. Pat. Nos. 3,226,447; 2,608,591 and 2,473,990 together with United Kingdom Pat. No. 778,642 point out these variations in results as to specific reactants and products utilizing varying catalysts and conditions. U.S. Pat. No. 3,366,698, provides disclosure of a specific prior art process wherein zirconium tetrachloride is the catalyst in a para-chlorotoluene/C1.sub.2 reaction. The commercial scarcity of zirconium tetrachloride however, increases the cost of the process and accordingly dampens its commercial competiveness.
It is an object of this invention to provide an improved process for the production of dichlorotoluenes. It is also an object of this invention to provide an improved process for the production of dichlorotoluene containing a very high percentage of 2,4-dichlorotoluene. Another object of the present invention is to provide an improved process for the production of substantially pure 2,4-dichlorotoluene. An additional object of the instant invention is for the production of a dichlorotoluene product that contains a substantially high yield of 2,4-dichlorotoluene, of high purity, and with substantially less amounts of less desirable dichlorotoluene isomers, i.e. 3,4-dichlorotoluene and 2,5-dichlorotoluene, and of trichlorotoluene. A further object of this invention is to provide an improved process for the production of dichlorotoluene at a lower commercial cost.
The process of the present invention is carried out by contacting parachlorotoluene with chlorine in the presence of a catalytic amount of antimony trichloride catalyst. Since antimony pentachloride is generally present in some amount in a commercial antimony trichloride composition and further since antimony trichloride may react in the chlorination process to form the pentachloride, a distinction between the trichloride and pentachloride in this process would be inappropriate. Accordingly, it is understood, that use of the term antimony trichloride catalyst includes a composition which may comprise antimony pentachloride.
More particularly, the process of the present invention comprises contacting liquid parachlorotoluene with gaseous chlorine in a mole ratio of about 0.5 to about 1.5 in the presence of less than about 1.0 percent of a catalyst comprising antimony trichloride, antimony pentachloride and mixtures thereof, until completion of the reaction. The reaction product is thus treated with a base and the catalyst is removed by separation or filtration. The resulting product is then fractionally distilled to separate the dichlorotoluenes from unreacted parachlorotoluene and any trichlorotoluenes that may be present. The product recovered is substantially pure 2,4-dichlorotoluene, by substantially pure meaning a product containing in excess of 95% by weight of 2,4-dichlorotoluene.
Only a small amount of antimony trichloride catalyst need be present in a reaction mixture to increase the relevant amount of 2,4-dichlorotoluene that is formed. As little as 0.05% of the catalyst, based on the weight of parachlorotoluene, will bring about a substantial increase in the 2,4-dichlorotoluene content of the chlorination product. There appears to be no advantage in using more than about 3.0% by weight of the catalyst, the preferred range being from about 0.05 to 1.0% by weight catalyst.
While the antimony trichloride catalyst is ordinarily preferably used as the sole chlorination catalyst in the chlorination of parachlorotoluene, it may also be used in combination with catalysts such as iron, ferric chloride or zirconium tetrachloride. The chlorination may, for example, be carried out by adding a mixture of the antimony trichloride catalyst and iron or zirconium tetrachloride or other catalyst to the reaction mixture or by carrying out the chlorination of parachlorotoluene in the presence of the antimony trichloride catalyst in an iron vessel. The antimony trichloride catalyst has been found to be effective in promoting the formation of 2,4-dichlorotoluene in the presence of other catalysts.
The chlorination of parachlorotoluene may be carried out by procedures that are well known in the art. For example, chlorine may be added to a reaction mixture containing parachlorotoluene and the chlorination catalyst until the increase in the weight of the reaction mixture, or the specific gravity, indicates that the desired amount of chlorine has reacted with parachlorotoluene. In the practice of the present invention the chlorination is usually continued until the reaction is complete. The reaction product generally contains in excess of 85% of dichlorotoluene and relatively small percentages of unreacted parachlorotoluene and trichlorotoluenes. The product of the chlorination is then treated with a base to remove the catalyst. Typical bases operable herewith include ammonia, organic amines, alkali and alkaline earth hydroxides, carbonates and bicarbonates. Preferred bases are annydrous ammonia, sodium carbonate and organic amines. After treatment with a base the catalyst, or catalyst base complex, may be removed by separation or filtration. The dichlorotoluene fraction, which may be separated from the parachlorotoluene and trichlorotoluenes by fractional distillation or other known technique, contains a high percentage of 2,4-dichlorotoluene, in excess of 95%, the remainder being 3,4-dichlorotoluene.
The chlorination reaction may be carried out at temperatures in the range of from about 0.degree. C to about 100.degree. C, with about 20 to about 70.degree. C the preferred temperature range. Below 0.degree. C the reaction takes place too slowly to be of commercial interest. At temperatures above 100.degree. C there is a tendency for side-chain chlorinated by-products and other by-products to be formed. Since chlorination is an exothermic reaction, external cooling may be required to maintain the reaction temperature in the desired range.
The rate at which chlorine is added to the reaction mixture does not have an appreciable effect on the yield of dichlorotoluene or the isomer distribution in the product.